High altitude fuel system



N. B. DEWEES HIGH ALTITUDE FUEL SYSTEM March 13, 1956 4 Sheets-Sheet 1 Filed Jan. 5, 1951 March 13, 1956 a DEwEEs HIGH ALTITUDE FUEL SYSTEM 4 Sheets-Sheet 2 Filed Jan. 5, 1951 a L i 2 mm m r O March 13, 1956 N. B. DEWEES HIGH ALTITUDE FUEL SYSTEM 4 Sheets-Sheet 3 Filed Jan. 5, 1951 March 13, 1956 B, D w s 2,737,897

HIGH ALTITUDE FUEL SYSTEM Filed Jan. 5, 1951 4 Sheets-Sheet 4 fnz/erzjf jybrman, 5. .pewees' HIGH ALTITUDE FUEL SYSTEM Norman Dewee's, Willoughby, (ihio, assignor to iii-org- Wamer Corporation, Chicago, 111., a corporation of Illinois Application January 5, 1e51, Serial No. 204,5es 11 claims. (c1.103-s7 This invention relates to a liquid pumping system, and more particularly to a fuel system for use in high altitude type aircraft. a 7

Because the vapor pressure of the high octane fuel used in aircraft is relatively high, that is, on the order of six to seven pounds per square inch at 100 F., as I the aircraft rises from ground level to higher altitudes where the surrounding air pressure becomes low and often lower than the vapor pressure of the fuel,- the fuel is subject to boiling and vaporization. This causes formation of bubbles in the fuel tank and pump inlet and it renders exceedingly difficult pumping of the fuel from the tank to the aircraft motor. To some extent, this undesirable condition can be mitigated by pressurizi'ng the fuel in the tank,- but this is considered dangerous and is particularly undesirable in military aircraft. Another and more generally employed system is that illustrated, for exampie; in Burns Patent No. 2,513,992 wherein a propellertype' pump is employed in tandem with a centrifugal pump, the propeller agitating the fuel and causing return to the fuel tank of air bubbles and also of considerable fuel. This propeller-type pump provides a supply of fuel to the centrifugal pump which is substantially free of air and vapor bubbles and thus the centrifugal pump does not fail by cavitation and is able to' develop the required pressure to the main fuel pump associated with theaircraft carburetor or fuel injection system.

This type of arrangement is obviously not eflicient since a considerable portion of the fuel is returned to the fuel tank and, also", because of the unavoidable low output of the propeller-type pump} the fuel supplied to the centrifugal pump is still at or very near the boiling point so that the performance of the centrifugal pump is limited by the adverse inlet conditions. Consequently, a: third or main fuel pump" is often required to develop the necessary, ultimate injection pressure; Furthermore, the efficiency continues to drop as the surrounding air pressure drops and thereby the practical operational ceiling of the aircraft isdefinitely limited. This complicated system naturally is' heavier and more expensive than a system wherein a centrifugalpump would be able to supply thefinal fuel pressure.

An object of the present invention is to provide a new and improved liquid pumping'psystem and more particularly a fuel system for high altitude aircraft.

A further object of: the present invention is to provide a fuel system of such increased performance, efficiency andreliability that only two pumping'stages are required.

A further object of the present invention is to provide a simplified, two-stage fuel system which is capable of pumping fuel at high pressure from a supply of boiling fuel.

A further object of this invention is to provide a fuel system incorporating improved thrust balancing and lubricating arrangements. 7

In accordance withone embodiment of this invention, the fuel system comprises anaxial flow type pump'connoted in tandem witha-centrifugalpump, the axial flow United States Patent 2,73 7,897 Mewa 2 pump being arranged to develop of itself sufficient pressure to cause any occluded vapor and air bubbles substantially to return to solution in the liquid portion of the fuel, thereby supplying essentially liquid fuel under moderate pressure to the inlet of the centrifugal pump. In accordance with this invention, no arrangementis made, her is any such arrangement necessary, for returning occluded fuel vapor or air bubbles to the fuel tank. Accordingly and because of favorableinlet conditions provided at the centrifugal pump, the eflicieilcy of the system is substantially greater than heretofore achieved with prior art systems. I p

In accordance with a second embodiment ofthis invention, two or more axial flow, impellers may be mounted in tandem on the impeller shaft to provide increased pressurizing of the fuel. p In accordance with a third embodiment of this invention, separate, coaxially arranged shafts may be employed for the axial flow impeller and for the centrifugal flow impeller, whereby the same may be rotated at different speeds. In similar manner where two ormore axial flow impellers are employed, these may be driven at different speeds. w Other objects and advantages of the presentinvention will be apparent from the following detailed description taken in conjunction with the drawings' whereinz a a Fig. l is a fragmentary; side elevation of the fuel system' of this invention incorporated in a fuel tank; p Fig. 2 is an axial, sectional view of a fuel system constructed in accordance with one embodiment of this invention; v v v p I p Fig. 3- is a transverse, sectional view tal'cje n substantially along the use 3 or Fig. 2 and showing to d antage the arrangement of the centrifugal flow pump stage;

Fig. 4 is'a' transverse,- sectional view taken substantially along the nae 494 of Fig", 2 and showingto advantage the arrangement of the axial flow pump stage of Fig. 2';' Fig. 5 is a detail, cfos's-section o'fa blade ofthe axial flow impeller, showing particularly th sh p leading' edgel; f a 1 Fig. 6" is a fragmentary; axial, sectional view p system ponstru'cted in accordance with asec'ofid dimen't' er this invention and having" two" tandem-mounted axiarflow impellers; and I, v,

Fig.7 is a fragment y, axial, sectional viewof "e1 system constructed in accordance with a third embodi men't of this invention. p j p z Referring new to the drawings, and particularly to amer, it will be seen that the fu'ell pumping system of this invention may comprise a' turbine driyilf fuel pump 10" mounted a suitable aperture 11 formed in the lower side wall 12 of a fuel'tank 1 3 sp thatthe fuel inlet ports'ilS' of the pump are always immersed in fuel in the tank. A suitable mounting flange; 16f is1pr o vided' on the pump hanging 17 for rn ounting' the p'ump in thetank. Output port 18 the pump, lflli's connected inside thetank to a fuel supply line 1 which may, in turn, be connected to the device (not shown) tobe supi plied' with fuel'by the pump. The driving injech m whicliiin the' embodiment illustrated anjair d vein turbine 20, but whieh may beany other suitable p wet source, is mounted below the" tank and turbine air nlet; port 21' is connected by line 22 to a" suitable source of air pressure not shown). j W 4 Referring now to Fi'g.'2',it will be'seen that the turbine 20'is'arranged'to drive a shaft 25Which is journalle'd the pump housing, and on whichis' mounted, adjacent the end' nearer the turbine 29, an axial'flow impeller 26. Adjacent the upper end of'shaft'ZS, that is, the end re mote from the,t urbine 26; there isimoun'ted a centrifuga'l flow impeller 27.. The; axial flow impeller 26 is mounted in the lower end of g n-annular chamber: 28 forme'd'in the pump housing, and fluid is admitted to theu'nder side of the axial flow impeller through the inlet ports formed in the housing 17 of the pump, as before described. Chamber 28 converges sharply immediately above the point at which the axial flow impeller 26 is mounted to form an annular throat 28a leading to the inlet of the centrifugal impeller 27 which is, in accordance with conventional practice, located in the center of a volute or scroll-shaped centrifugal pumping chamber 30 formed in the upper end of the pump housing. Cham ber 30 connects at its output or discharge end with the discharge port 18 of the pump 10.

In the operation of this pump, fuel admitted to the axial flow impeller through the inlet ports 15, or fuel and air and vapor, is compressed by the axial flow impeller so that the pressure of the fuel is increased by a suflicient amount to cause the air and vapor to return into solution, in efiect, in the liquid portion of the fuel whereby the fuel supplied to the inlet of the centrifugal impeller is wholly liquid. In practice, it has been found this requires that the axial flow impeller pump be capable of raising the pressure of the fuel approximately five pounds at sea level. The centrifugal pump then further compresses the fuel, raising the pressure thereof to a value which may be on the order of 500 p. s. i. and the high pressure fuel is discharged through the outlet port 18.

Contrary to the usual construction of such fuel systems for high altitude type aircraft, no means is provided in this pump for returning to the tank occluded air and vapor, the rate of fiow and the pressure of the fuel discharged from the axial flow pump being selected with reference to the ultimate desired pressure of the fuel pump so that the centrifugal pump may operate without detrimental cavitation or vapor lock. Accordingly, whereas in prior known devices of this general character, anywhere from 25 to 40% of the power required to operate the system was devoted solely to the purpose of removing from the fuel occluded air and vapor, this loss is wholly avoided by the present invention and instead this heretofore lost power is utilized to actually raise the pressure of the fuel in the first or axial pumping stage. Furthermore, in prior known devices, the efficiency of the centrifugal pump has suffered because of adverse inlet conditions, but in the present invention inlet pressures are adequate to give improved pump efficiency. Consequently, it is possible to use a centrifugal pump designed to deliver fuel under sufiicient pressure so that a final fuel pump, as is customarily employed with prior art devices, beyond the centrifugal booster pump is unnecessary in most applications. in cost and weight are obviously very important.

' Referring now particularly to the construction of the air driven turbine shown in Fig. 2 used for rotating the shaft 25, it will be seen that air entering the turbine through the port 21 is directed through a stator 31 of the turbine, the blades of which are curved and set at an angle with respect to the direction of flow of the air stream so as to direct the air stream against the blades of a turbine rotor 32, keyed to the lower end of shaft 25, with maximum efficiency. After the air has passed through the rotor blades, it escapes through an outwardly extending duct or passage 33. While a turbine is shown as the driving mechanism for the shaft 25, it will be understood that other driving mechanism may be employed where the high rotational speeds available from a turbine is not required, or where a suitable source of air pressure to operate the turbine is not available. However, in the use for which this system is primarily intended, it is necessary that the fuel pump deliver pressure at a very high value and at the same time it is essential in view of the intended aircraft use that dimensions and weight of the pump be kept at a minimum. Therefore, in order to develop the required power and rate of flow, a turbine is most suitable.

' Referring now to the axial flow pump section of this The resultant savings fuel pump, it will be seen that the outer wall of the lower section of the pump housing 17 immediately above the flange 16 comprises a plurality of struts 34 and integral flow guide vane members 34:; which are spaced circumferentially and define therebetween the inlet ports 15 to the lower portion of the chamber 28, that is, the portion of the chamber beneath, as viewed in Fig. i), the axial flow impeller 26. Vane members 34a prevent pre-rotation of the fuel and thereby enable the axial flow impeller to act more forcibly on the fluid which in turn makes possible a higher axial flow pressure. The base of chamber 28 actually extends below the level of the mounting flange 16, which as aforesaid is mounted in the lower side wall of the fuel tank 13 so that any fuel in the tank is communicated through the ports 15 to the chamber 23. Normally, in the operation of this pump, the tank is sufficiently filled with fuel so that the entire pump is immersed in fuel and, of course, the lower end of the chamber 28 is completely filled with fuel. Where self-sealing tanks are employed having inner wall thicknesses as indicated at 13a which may be on the order of one-and-on-half inches or more, a pocket is formed at the point the pump mounting flange is attached and this pocket normally remains filled until the tank is exhausted.

The axially apertured, lower central portion 17a of the housing 17, through which portion the lower end portion of shaft 25 is journalled, is of substantially greater inside diameter than the portion of the shaft 25 disposed therewithin and a sleeve 35 is mounted in this lower central portion 17a in the axially extending aperture designated 36, the sleeve 35 being retained in the aperture 36 at its lower end by a small, inwardly extending shoulder or flange 37. The central portion 17a of housing 17 is widened sharply at its upper end to define an annular recess 38 and to receive a retaining nut 4 which is threaded on the shaft 25. Retaining nut 40 bears against the lower side of the hub 41 of the axial flow impeller, the hub 41 being keyed to the shaft 25 at this point. The upper side of the hub 41 bears against a flow guide collar 42 which is mounted on the shaft 25. Flow guide collar 42 is provided with an inwardly extending shoulder 43 adjacent its lower end which bears against a suitably formed shoulder 44 provided at the lower end of an enlarged portion 25a of shaft 25. The flow guide collar 42 is tapered inwardly at its outer periphery so that the upper end thereof converges toward the axis of shaft 25 and meets the lower end of a converging portion 25b of shaft 25 to provide a substantially continuous, converging surface. The periphery of the collar 42 and the periphery of the converging portion 25b define thus the inner wall of the upwardly converging chamber 28 of the pump, and lead through throat 28a to the inlet or eye of the centrifugal pump impeller section of this fuel pump. The outer wall of this portion of the chamber 28 is defined by a converging outer wall 17b of the pump housing. This wall 17b is continuous and converges somewhat more sharply than the outer surfaces of the collar 42 and shaft portion 25b so that the throat grows narrower in crosssectional area as it approaches the inlet of the centrifugal impeller. A chamber of relatively large size is required at the inlet to the axial flow impeller since the fuel here contains occluded air and vapor; however, as these are absent at the discharge side, desirable flow conditions are provided by the converging throat. Mounted outside of the struts 34 is a screen or filter 45 which prevents foreign matter from reaching the centrifugal pump and the axial flow pump portions.

As clearly shown in Fig. 2, the impeller 26 is positioned within the entrance to the lower end of the annular, upwardly converging portion of chamber 28. The blades of the impeller are selected to have the maximum radial extent feasible and rotate with their tips just out of contact with the inner wall of the chamber 28a. There is thus no return path,- as in prior art devices, permitting return of vapor and occluded air to the inlet of the pump.

It has been recognized for some time that an axial flowtype pump has the ability, in general, to pump closer to the boiling point than a centrifugal pump. However, in order to take maximum advantage of this characteristic of an axial flow pump, it has been found in accordance with the present invention desirable to modify the axial flow blades over the design conventionally employed. Referring now to Fig. 5, which is a cross-sectional view taken through a blade 46 of the axial flow impeller, it will be seen that the leading edge 46a of the blade is made very sharp. This is contrary to conventional low speed impeller practice with inlet pressures weil above the boiling point. In the conventional construction and leading edge 46a would be more like an air foil, that is, rounded. This conventional construction permits a greater divergence of operation from the design point without causing cavitation. However, in the use for which the present impeller is intended, cavitation is already present and, therefore, the fluid being pumped does not necessarily follow the back side of the blade. In practice it has been found that a blunt edge under these circumstances merely causes further bubble formation because of the impact forces and results in increased turbulence and the formation of low pressure areas. Therefore, in accordance with the present invention the leading edge is made sharp and the entire blade is actually a very thin cross-section. The angle of the blades and the number and extent of the blades is selected with reference to the pumping conditions to be encountered so that a moderate increase in pumping pressure is provided,

The centrifugal flow impeller 27 is mounted adjacent the upper end of the shaft 25, the hub 50 of the centrifugal flow impeller being threaded on an enlarged portion 250 of shaft 25 which is suitably shouldered so that by tightening the centrifugal flow impeller in position the impeller is securely mounted on the shaft. The threading is made opposite to the direction of rotation of the shaft 25 and thereby as the shaft rotates the impeller is prevented from becoming loose on the shaft.

A pump of this design is subject to very large thrust pressures. Thus, while an upward thrust is produced by the rotor of the air turbine tending to move the shaft 25 upwardly, particularly the axial flow impeller, but also the centrifugal flow impeller, in moving the fluid being pumped upwardly exert a large downwardly directed force. In a practical embodiment of this pump, a measured net thrust downwardly of approximately 800 pounds was found to be present. Such a thrust applied to the best available bearings at the very high rotational speeds, that is, speeds upwards of 20,000 R. P. M., would result in undue wear and a very short operational life. In accordance with the present invention, however, provision is made for utilizing the pressure developed by the centrifugal impeller pump stage to counteract and offset this downward axial thrust whereby the thrust forces on the shaft 25 are substantially balanced.

Referring again to Fig. 2, it will be noted that the enlarged portion 25c of the shaft 25 on which the centrifugal impeller 27 is threadedis provided at its up per end with a sheuld'cr' or flange 51 against which the upper side of the hub 50 of the impeller bears. The upper side of the flange 51 is normally spaced from the inner under side of an annular bearing 52 which is mounted in the Upper portion of the housing 17 on an inwardly extended shoulder or ledge 53 of the housing. The upper side of-the outer portion of the bearing ring 52 is clamped securely in position by a housing end cap or cover 54 suitably bolted to the upper end of the housing. The upper end-of shaft 25 has secured thereto on a reduced portion 25d a thrust washer or di sc 55, the hub 55a of the washer resting, on a shoulder 56 formed on the shaft 2'5 6 V at the point at which the shaft 25 is reduced to provide the smaller diameter, upper, terminal portion 250! and a retaining nut 57 is threaded on the outer end bf the terminal portion 25d so as to clamp the thrust washer 55 against this shoulder 56. The thrust washer 55 is thus rotatable with the shaft 25 and firmly secured thereto for longitudinal or axial movement therewith. The thrust washer 55 has a shoulder or ridge 55b formed at the under side of the outer periphery, which ridge 55b bears against the upper side of the bearing 52. Because the ridge 55b extends downwardly beyond the main body portion of the lower surface of the thrust washer 55, there is provided an annular pressure chamber 60 which communicates at its radially inner end through axially extending grooves 61 formed in the inner surface of bearing ring 52, ora suitable clearance provided between the bearing ring 52 and the periphery of the shaft 25, with the upper end of the centrifugal pump impeller chamber 39; In this portion of the volute chamber 30 a relatively high pressure, approaching that of the discharge pressure of the centrifugal pump, is present during the operation of the pump. This pressure being communicated to the chamber 6% tends to move the thrust washer55 upwardly. However, if the shaft 25 moves upwardly then fluid in the chamber 68 is permitted to escape or to pass by the ridge 51% into a relatively low pressure chamber designated 62 and defined by the upper side of thrust washer 55 and the inner side of the end plate or cap 54. The fluid escaping past the ridge 55b is replaced by more fluid from the volute chamber 36, but the pressure in annular chambe: 66 is lower than before shaft movement because of the pressure drop through the grooves 61 and the clearance between the bearing ring 52 and the peripheryof the shaft 25. Thus, movement of shaft 25 downwardly sufficiently to decrease this escape passage causes pressure to accumulate in the chamber 60, and this upwardly effective pressure force exerted against washer 55 has been found adequate, by a proper selection of the lower surface area of thrust washer 55, to compensate for the downward thrust produced by the rotor of the turbine, the axial impeller and the centrifugal impeller on the shaft 25. In testing a pump constructed in accordance with this fea ure of the present invention, it has been determined that axial displacement of the shaft 25 provided with this thrust balancing arrangement is actually very slight and that a substantially stable operating condition is maintained, the passage past the shoulder 55b being maintained at a fairly definite size so that a definite upwardly directed pressure is present in the chamber 60 to offset the thrust downward on the shaft 25. Likewise the fluid fiow through the clearance between shaft 25 and bearing ring 52 and on past ridge 5551 has been found adequate to lubricate the bearing ring 52 at its contact with shaft 25 and ridge 55b.

Because of the high rotational speeds present, it is desirable to make the pump self-lubricating and this is accomplished in accordance with the present invention as indicated above and by bleeding the fuel escaping into the chamber 62 through an axially extending passage 63 formed in the shaft 25 and utilizingthe fluid to lubricate the journal contact areasof shaft 25. Passage 63 extends downwardly toward the lower end of the shaft 25' tocommunicate with radially extending passages 64, locatedadjacent the lower end of shaft 25; Here it will be noted pressure is communicated by these passages 64 to a chamber 65 defined by, at its outer side, a Sylphon or bellows-type seal 66. The chamber 65 is closed'at its lower end, as will be described hereinafter, and therefore pressure reaching the chamber 65 can escape only upwardly through either the clearance provided between the periphery of shaft 25 and the inner side wall of the sleeve 35 or through suitable axially extending grooves 6 7 provided in this area in the inner side of sleeve 55. The mid portion of sleeve 35 is annularly recessed as indicated at 70 andthe upper portion may bi: similarly 7 grooved as was the lower portion or again suitable clearance provided to permit passage of fluid upwardly along the periphery of the shaft to provide lubrication.

Adjacent the upper end of the sleeve 35 the shaft 25 is provided with passages 71 which communicate at their lower ends with a generally annular recess 72 provided by slightly recessing the upper terminal end of the sleeve. The passages extend generally upwardly but at first inwardly and then outwardly so as not to weaken the shaft periphery in the area on which the axial flow impeller 26 is mounted. At their upper ends the passages 71 communicate with the inlet portion of the throat 23:: leading to the eye of the impeller 27. The upper end of the passages 71 are thus communicated to a zone of relatively low pressure and, consequently, there is a substantially continuous flow of fluid from the chamber 61?. at the upper end of the pump housing down through the central passage 63 formed in the shaft 25, out through the passsages 64 and upwardly along the periphery of shaft 25 lubricating the contact surfaces of the shaft and the sleeve 35. The annular clearance 72 is comparable to that between the shaft 25 and the sleeve 35 and no grooves are provided. Thus, most of the lubricating fluid is forced through the passages 71 rather than through the annular clearance 72 from whence it would go into the inlet of the first stage. The lubricating fluid will be warmed slightly after passing over the bearings and might flash into undesirable quantities of vapor if allowed to wholly discharge into the first stage inlet.

Referring now in more detail to the assembly of the bellows seal 66, it will be seen that an annular mounting plate 73 is mounted on the lower end of the central portion 17a of the pump housing 17 by bolts 74 and to the inner edge of this mounting plate is secured the upper end of the bellows 66. The bellows 66 is thus stationary with respect to the shaft 25'. The lower end of the bellows 66 has attached thereto a bearing disc '75 of a suitable low friction, dimensionally stable material such as carbon, the lower side of which disc is provided with a ridge 75a which bears against the upper side of a seal disc 76 mounted on the shaft 25, being sandwiched between a shoulder 77 on the shaft 25 and the upper side of the rotor 32 of the turbine. Helically coiled springs 80 are interposed between the upper surface of the bearing disc 75 and the under side of the mounting disc 73 to urge the bearing disc 75 downwardly and maintain a suitable seal at the ridge 75a with the upper surface of the seal disc 76. It will be noted that the outer portion of the seal disc 76 is flanged upwardly and then outwardly to overlap an annular wall portion 81 of the turbine housing. The purpose of this is to deflect any fuel leaking past the seal 75a into the annular seal chamber bounded on the inside by the wall 81 and from which the leakage may be drained to the exterior of the turbine housing by suitable holes.

From the foregoing it will be evident that this seal assembly permit axial displacement of the shaft 25 while maintaining a suitable seal so that adequate pressure will be developed in the chamber 65 for the purpose of lubricating the shaft.

Design requirements for some aircraft require that the fuel system be capable of functioning satisfactorily at altitudes even in excess of 50,000 feet. At 50,000 feet the ambient atmospheric pressure is on the order of 1.69 p. s. i. a. The absolute vapor pressure of standard fuel for many types of military aircraft is approximately 9 p. s. i. at lower altitudes. As the plane climbs to higher altitudes the fuel will boil as the pressure over the fuel in the tank reaches the absolute vapor pressure of the fuel, i. e. at about 14,000 feet altitude for the vapor pressure given. This point normally is the most difficult to pump because any air dissolved in the fuel is coming out at this time and contributing to the boiling. From the boiling altitude on up the fuel continues to boil because of the decreasing pressure, with the more volatile portions of the fuel boiling off first, thus changing the composition of the remaining fuel so that its absolute vapor pressure is always essentially equal to the pressure over the fuel in the tank. At higher altitudes the density of the evolved vapor is reduced approximately as the pressure over the fuel is reduced. Thus, at 50,000 feet the volume occupied by one pound of fuel is approximately five times the volume occupied at the initial boiling point. This might make pumping at high altitudes exceedingly difilcult except that normally the required fuel flow, pressure and airplane climb rate are greatly reduced at the high altitudes. However, in order to meet the problem of sustained performance at high altitude, in accordance with the present invention one or more additional axial flow impellers may be mounted in tandem on the shaft 25' to provide increased pressurizing of the fuel supplied to the centrifugal flow impeller 27.

Referring now to Fig. 6 wherein identical reference numbers have been employed to those used in the preceding figures with the exception of reference numeral 82 designating the second axial flow impeller, it will be seen that this second axial flow impeller may be conveniently mounted on the shaft 25 above the axial flow impeller 26 by forming the impeller 82 integrally with the flow guide collar 42. It would, also, be feasible to use a separate impeller mounted on the collar 42 by suitable retaining means having a surface configuration conforming to the desired throat surface. The axial flow impeller is desirably provided with blades having a cross-sectional configuration such as that illustrated in Fig. 5 and the tips of the blades rotate in close proximity to the inner wall of the housing section 17 b. A plurality of stationary guide vanes 33 are mounted on the inner side of housing 17b and extending into the discharge flow path of the first axial flow impeller to establish a suitable inlet condition to the second axial flow impeller. These vanes are located radially and equally spaced circumferentially and may comprise substantially fiat sheets.

With this arrangement, the first axial fiow impeller 26 raises the pressure of the fuel as supplied to the second axial flow impeller 82 which, in turn, raises the pressure of the fuel as supplied to the centrifugal impeller. The net increase is greater than that feasible with a single axial flow impeller.

In some cases it is desirable that the axial flow impeller and the centrifugal flow impeller be rotatable at different speeds and particularly at diiferent speeds as the atmospheric pressures varies. Thus the speed at which the axial flow impeller will pump fuel most efficiently at sea level is less, ordinarily, than the speed at which it will pump most eificiently at higher altitudes and especially as the boiling point of the fuel is reached. Accordingly, if means be provided for increasing the speed of rotation as the fuel becomes more filled with vapor and bubbles, then the normal decrease in the discharge pressure of the axial fiow pump at higher altitudes may be substantially avoided. Also, ordinarily the maximum efliciency speed of an axial flow impeller of the type illustrated in Fig. 2 will differ from the maximum efliciency speed of a centrifugal flow impeller designed for cooperation therewith. Accordingly, with a single driving shaft, either the axial flow impeller is operated beyond its maximum efiiciency speed or the centrifugal flow impeller is operated below its maximum efficiency speed. It will be understood that particularly with axial flow liquid pumps of this type, if the pump is operated beyond its maximum etficiency speed under certain conditions, the flow rate or discharge pressure actually drops although the driving power required increases with increasing rotational speeds.

Also, it is sometimes desirable where two axial flow impellers are used in tandem as shown in Fig. 6 to operate the second impeller at a lower rotational speed than the first impeller, since the vapor occlusion problem is substantially less at the inlet of the second axial flow impeller. That is to say, the fuel is more nearly solid. In order '9 to. accomplish this it is then necessary to employ separate coaxially rotating driving shafts.

Referring to Fig. 7, there is illustrated, in. simplified form, a fuel pump of the type shown in Fig. 2 but wherein two separate driving shafts 90 and 91, arranged concentrically, are employed to drive separately the axial flow impeller and the centrifugal flow impeller. Shaft 90' may be rotatably journalled in shaft 91 as shown. The inner driving shaft 90 drives the centrifugal flow impeller and the outer driving shaft 91 drives the axial flow impeller. In order to provide for turbine drive of these two shafts, a two-stage turbine arrangement is employed. In this arrangement there is provided a stator 94 and a rotor 93 which is keyed to the outer shaft 91. A second stator is mounted below the: rotor 93 and a second rotor 95 is keyed to the end of shaft 90, which end protrudes beyond the terminal end of shaft 91. Air is admitted through passage 96' through the upper side of the first turbine stage through the stator 92 and is discharged through passage 97 to the turbine discharge. Since the first rotor 93 receives air at the greater density, the air velocity and hence rotor and shaft 91 rotational speed will tend to be less than that of the second rotor 95 and inner shaft 90 to which the centrifugal flow impeller is secured.

Where the centrifugal flow impeller whichv is. further from the turbine than the axial flow impeller is to be rotated at the higher speed, the air inlet must, be from the upper side. This, however, increases the thrust problem, since the thrust produced on the two driving shafts will; nov. be similarly directed to the thrusts produced by the axial flow and the centrifugal flow impellers. This additional thrust may, of course, be compensated by increasing the area of thrust washer 55 defining the upper side of chamber 60. the higher speed rotating shaft to drive a second axial flow impeller, in which case the centrifugal flow impeller is either mounted on the higher speed shaft or the lower speed shaft, as desired. A so, it will. be evident that while but two turbine stages have been shown,

it is feasible to employ more than two turbine stages where, for example, it is desired to employ two axial flow pumping stages and a centrifugal flow pumping stage, all operated at different speeds and mounted on three separate, coaxially disposed shafts. The separate turbine stages, in effect, provide an automatic distribution of driving power in accordance with the requirements of the separate pumping shafts. By selecting the turbine rotor configurations of the several rotor stages this distribution may be in part predetermined in advance in accordance with the desired pumping characteristics of the separate pumping stages. 7

Where herein the various parts of this invention have been referred to as being located in a right or a left position, or an upper or a lower position, it will be understood that this is done solely for the purpose of facilitating description and that such references relate only to the relative positions of the parts as shown in the accompanying drawings.

What is claimed is:

l. A high altitude pumping system for pumping volatile fuel comprising an axial flow pump, said pump having at least one group of impellers and a centrifugal pump, both of said pumps being connected in tandem and mounted on separate coaxial shafts, said axial flow pump being arranged to supply fuel under pressure to the inlet of the centrifugal pump means for driving said shafts so that said axial flow pump rotates at one speed and said centrifugal pump rotates at another, and axial thrust balancing means utilizing a portion of the pressure developed by said centrifugal pump to compensate for the resultant axial thrust developed by both said pumps, said thrust balancing means comprising disc means mounted on said driving means and having one surface thereof subjected to said pressure.

Itis also equally feasible to use 2.1 A high altitude pumping system for volatile fuel comprising an axial flow pump having a plurality of groups of impellers and a centrifugal pump, said pumps being connected in tandem and mounted on separate coaxial shafts, means for driving one shaft at one speed and the other at another speed, said groups of impellers being spaced apart so that each group increases the pressure of the fuel supplied to-the inlet of the centrifugal pump, and axial thrust balancing means utilizing a portion of the pressure developed by said centrifugal flow pump to compensate for the resultant axial thrust developed by said pumps, said thrust balancing means comprising disc means mounted on said driving means and having one surface thereof subject to said pressure.

3. A high altitude fuel pumping system for pumping volatile fuel comprising an axial flow pump, a centrifugal pump, and separate coaxial driving shafts for retating said axial flow pump at one speed and said centrifugal pump at another speed, driving means mounted on said coaxial shafts in tandem with said pumps, and axial thrust balancing means utilizing a portion of the pressure developed by said centrifugal flow pump to compensate for a resultant axial thrust developed by said pumps, said thrust balancing means comprising disc means mounted on the shaft driving said centrifugal pump and having one surface thereof subject to said pressure so as to develop a force in opposition to said resultant axial thrust.

4. A pump for handling fluids at temperatures at or above the boiling point. of the fluid comprising, a casing defining a volute chamber having a peripheral discharge, driving means mounted in said casing and extending axially thereof, a centrifugal impeller mounted on said driving means, anintake port in said casing, an axial flow impeller for inducing flow of fluid axially from said intake port and for discharging fluid towards said centrifugal impeller, said axial flow impeller being mounted on said driving means and driven at a different speed than that of said centrifugal impeller, flow guide defining means between said centrifugal impeller and said: axialimpeller, saidflow guide defining means reducing the cross sectional area of the flow of fuel from said axial flow impeller so that said fuel communicates only with the inner regions of said centrifugal impeller, said flow guide defining means forming a part of said driving means, means for utilizing a portion of the discharge of the centrifugal flow impeller for balancing the axial thrust developed by said impellers, means for utilizing a portion of the discharge of said axial flow impeller for lubricating said pumps, and means for imparting torque to said driving means.

5. The combination as claimed in claim 4 wherein said means for utilizing a portion of the discharge of the centrifugal impeller for balancing the axial thrusts of said impeller comprises a disc mounted on one end of said driving means, means for communicating pressure to one surface of said disc, and means for permitting said pressure to escape from said surface to regulate the pressure thereat depending upon the thrust of said impellers.

6. The combination as claimed in claim 4 wherein said means for lubricating said pumps comprises flow path defining means formed within said driving means and communicating with a surface on said drive means.

7. The combination as claimed in claim 4 wherein said 1 driving means comprises a plurality of shafts mounted coaxially, said axial flow impeller being mounted on one of said shafts and the centrifugal flow impeller being mounted on another of said shafts.

8. A high altitude fuel system for pumping volatile fuel comprising an axial flow pump, a first drive shaft for said axial flow pump, a centrifugal pump having its inlet connected to the discharge side of said axial flow pump, a second driving shaft coaxial with said first shaft for said centrifugal flow pump, said axial flow pump being arranged to supply fuel under pressure to the inlet of the centrifugal pump, means for driving said first shaft at one speed and said second shaft at another speed, and axial thrust balancing means utilizing a portion of the pressure developed by said centrifugal flow pump to compensate for a resultant axial thrust developed by said pumps and said driving means, said thrust balancing means comprising disc means mounted on one of said driving shafts and having one surface thereof subjected to said pressure so as to develop a force in opposition to said axial thrust.

9. A high altitude fuel system for pumping volatile fuel comprising an axial flow pump, a first driving shaft for said axial flow pump, a centrifugal pump having its inlet connected to the discharge side of said axial flow pump, a second driving shaft coaxial with said first shaft for said centrifugal flow pump, said axial flow pump being arranged to supply fuel under pressure to the inlet of the centrifugal pump, a two-stage air driven turbine for driving said shafts, one of said stages operative to drive said first shaft at one speed and the other stage operative to drive said second shaft at another speed, and axial thrust balancing means utilizing a portion of the pressure developed by said centrifugal flow pump to compensate for a resultant axial thrust developed by both said pumps and said turbine, said thrust balancing means comprising disc means mounted on one of said shafts and having one surface thereof subjected to said pressure so as to develop a force in opposition to said resultant thrust.

10. A high altitude fuel pumping system for pumping volatile fuel comprising an axial flow pump, said pump having a plurality of groups of impellers and a first chamber of a generally annular configuration, said chamber having a plurality of circumferentially spaced partitions around the outer periphery thereof to define a plurality of inlet passages, and a second chamber communicating with said first chamber and converging to an outlet passage, said groups of impellers being disposed in said second chamber and operable to move fuel from said inlet passages to said outlet passage along a path generally longitudinal of said pump.

11. A high altitude fuel pumping system for pumping volatile fuel comprising an axial fiow pump, said pump having a first chamber of generally annular configuration, said chamber having a plurality of circumferentially spaced, inwardly extending, fiow guides positioned around the outer periphery thereof to define a plurality of inlet passages, and a second chamber communicating With said first chamber and converging to an outlet passage, a filter means disposed around the outer ends of said guides for enclosing said inlet passages, and impeller means disposed in said second chamber and rotatable to move said fuel from said inlet passages to said outlet passage along a path generally longitudinal of said pump.

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